My invention relates generally to a sheet metal screw and more particularly to a sheet metal screw with a head, a shank having a substantially long parallel section distally located from said head and a relatively shorter tapered section proximate to the head. The head has locking serrations on its underside.
It will be appreciated by those skilled in the art that sheet metal screws must be specifically designed. It will be further appreciated by those skilled in the art that sheet metal used to assemble products such as electrical appliances are becoming thinner and thinner. This increases the likelihood that the screw in the manufacturing of such appliances will cause the sheet metal to be stripped out as it is torqued into the sheet metal thereby creating a useless piece of thin sheet metal. Further, if the sheet metal screws are made as soft as the sheet metal to prevent the sheet metal from being stripped, the heads of the screw can easily be torqued off. To this end, there have been several attempts to design a screw that is useable in connection with thin sheet metal applications.
One such attempt is disclosed in U.S. Pat. No. 3,752,030, issued to Steurer on Aug. 14, 1973. Steurer discloses a section of shank which is not tapered followed by a gradually and progressively increasingly tapered section which will expand the pilot hole and gradually compress the metal surrounding the hole. Unfortunately, the long and gradually extending taper actually can create the stripping which Steurer seeks to prevent. Tapering causes the sheet metal to curl toward the head. If the taper extends over too great a length, the curling will strip the metal.
Another such attempt is disclosed in French Patent No. 79 17837 issued on Jan. 30, 1981 to Francois Klein The Klein patent discloses a head having, on its underside, radial ribs. Unfortunately, Klein was intended for use with wood and the ribbing can not prevent the type of stripping and improved attachment which is needed.
What is needed, then, is a sheet metal screw which has a taper which improves the attachment between the screw and the sheet metal yet prevents the stripping of the sheet metal by a screw having too great of a taper. Further, a screw is needed which can take advantage of the curl up which is created by the engagement of a tapered screw in sheet metal. This sheet metal screw is presently lacking in the prior art.